The rice aquaporins OsPIP2;1 and OsPIP2;2 alleviate toxicity of heavy metals and salinity and promote root stress avoidance via ROS signaling

Abstract Soil contamination with heavy metals and salinity threatens global crop production. Heavy metal and salinity stresses induce reactive oxygen species (ROS) burst. How ROS homeostasis and signaling are maintained under these stresses are not fully understood. Among the 11 genes encoding the Plasma-membrane Intrinsic Proteins (PIPs), we found that expression of OsPIP2;1 and OsPIP2;2 in rice roots was upregulated by exposure to heavy metals (Cd, Cu or Pb) and salinity. Both genes were mainly expressed in the outer cell layers and the stele of roots. OsPIP2;1 and OsPIP2;2 were permeable to H2O2. Knockout of both genes increased cytosolic ROS accumulation but decreased apoplastic ROS levels, resulting in a greater inhibition of root and shoot growth by heavy metals and salinity. The effect from knocking out both OsPIP2;1 and OsPIP2;2 was additive of single gene knockouts. The mutant phenotypes were rescued by chemical ROS scavenging, suggesting that the increased sensitivity to heavy metals and salinity in the knockout mutants is attributed to increased cytosolic ROS accumulation. Furthermore, knockout of both OsPIP2;1 and OsPIP2;2 impaired the propagation of systemic ROS signaling from stress-exposed to unexposed roots, compromising the ability to proliferate lateral roots in the stress-free zone, a stress avoidance strategy observed in wild-type plants. Taken together, OsPIP2;1 and OsPIP2;2 play dual roles in alleviating oxidative damage caused by heavy metals and salinity by transporting ROS out of the cytoplasm, and in transducing long-distance systemic ROS signaling to trigger root system remodeling and stress avoidance in response to localized stress.